In an EL element, holes and electrons, which are injected from electrodes opposed to each other, are bonded to each other in a light-emitting layer, whereby the resulting energy excites a fluorescent material in the light-emitting layer and luminescence of a color in accordance with the fluorescent material is effected, so that EL elements are now becoming increasingly popular as a self-luminous sheet-shaped display element. Among EL elements, an organic film EL display, which employs an organic material as a light-emitting material, exhibits relatively high luminescence efficiency such that luminescence of high intensity can be realized when voltage a little less than 10 V is applied, and emitting light with a simple element structure is possible. Accordingly, it is expected that the organic film EL display can be applied to a low-cost indication display having a simple structure, such as an advertisement device which indicates specific patterns by emitting light.
When a display using such an EL element is produced, patterning of a first electrode layer and an organic EL layer is generally carried out. Examples of patterning method of such an EL element include: a method of vapor-depositing a light-emitting material via a shadow mask; a method of coating selectively by inkjet; a method of destroying specific light-emitting colorants by UV radiation; a screen-printing method; and the like. However, according to the aforementioned conventional methods, it is not possible to provide an EL element which satisfies all the requirements such as high luminescence efficiency, high “yield” of light eventually obtained, simple and easy production process and highly minute and precise pattern formation.
As a solution for solving the problems described above, a method for manufacturing an EL element has been proposed, in which a light-emitting layer is formed by patterning by photolithography. This method dose not require a vacuum facilities and the like equipped with highly precise alignment mechanism so that it makes production of EL elements relatively easy in low cost, as compared with the conventional patterning method by vapor-depositing. On the other hand, this method is preferable because structures for assisting patterning or pre-treatments of a substrate is not necessary, as compared with the patterning method using inkjet. To form highly precise patterns, the method for manufacturing an EL element by photolithography is more advantageous and preferable than the patterning method using inkjet, considering a discharge precision of an inkjet head.
An example of a method of forming a plurality of light-emitting portions by such a photolithography is shown in FIG. 2.
As shown in FIG. 2(a), a first electrode layer 2 is formed in a pattern on a substrate 1, a first light-emitting portion 3 is formed in a pattern on the fist electrode layer 2, and a fist photoresist layer 4 is laminated on the first light-emitting portion 3.
Next, as shown in FIG. 2(b), a second light-emitting layer 5 is formed by coating the substrate with a second light-emitting layer forming coating solution. And a positive-type photoresist is coated on the entire surface of the second light-emitting layer 5, whereby a second photoresist layer 4′ is formed. Then, as shown in FIG. 2(c), only the portions which the first light-emitting portion 3 and a second light-emitting portion are to be formed, are masked by a photomask 6 and the remaining portions are exposed to ultraviolet.
The product is developed by a photoresist developer and washed, whereby the second photoresist layer 4′ of the exposed portions is removed, as shown in FIG. 2(d). Further, portions of the second light-emitting layer 5, which is bared as a result of removal of the second photoresist layer 4′ at the exposed portion, are removed. A second light-emitting portion 5′ coated with the second photoresist layer 4′, and the first light-emitting portion 3 coated with the first photoresist layer 4, the second light-emitting layer 5 and the second photoresist layer 4′ are obtained, as shown in FIG. 2(e).
Next, same as the pattern formation of the second light-emitting portion 5′, a third light-emitting layer forming coating solution is coated on the substrate, as shown in FIG.2(f), whereby a third light-emitting layer 7 as a film is formed. And a positive-type photoresist is coated on the entire surface of the third light-emitting layer 7, whereby a third photoresist layer 4″ is formed. Then, as shown in FIG. 2(g), portions which the first light-emitting portion 3, the second light-emitting portion 5′ and a third light-emitting portion 7′ are to be formed, are masked by a photomask 6 and the remaining portions are exposed to ultraviolet.
The product is developed by a photoresist developer and washed, whereby the third photoresist layer 4″ is formed in a pattern, as shown in FIG. 2(h). Then, the third light-emitting layer 7, which is bared as a result of removal of the third photoresist layer 4″ at the exposed portions, are removed. As a result, a third light-emitting portion 7′ coated with the third photoresist layer 4″ is obtained, as shown in FIG. 2(i). On the second light-emitting portion 5′, the second photoresist layer 4′, the third light-emitting layer 7 and the third photoresist layer 4″ are laminated. On the other hand, the first photoresist layer 4, the second light-emitting layer 5, the second photoresist layer 4′, the third light-emitting layer 7 and the third photoresist layer 4″ are laminated on the first light-emitting portion 3.
Finally, by peeling treatment using a photoresist peeling solution, the first light-emitting portion 3, the second light-emitting portion 5′ and the third light-emitting portion 7′ are bared, as shown in FIG. 2(j).
Thereafter, further processes including a process of forming a second electrode layer on each light-emitting portion are carried out, whereby an EL element which emits luminescence in the direction toward the bottom of the page in FIG. 2(j) is produced.
However, in the conventional method as described above, it is difficult to peel the photoresist layer quickly in the peeling treatment of the photoresist layer at the final stage from FIG. 2(i) to FIG. 2(j). Because a plurality of light-emitting layers and photoresist layers are laminated on the target photoresist layer to be treated, the area of the target photoresist layer to be treated, where the target photoresist layer is brought into contact with the photoresist peeling solution, is small. Accordingly, it takes a very long time for the photoresist peeling solution to act on the photoresist layer in the sufficient manner. When the substrate is exposed to the photoresist peeling solution for a long time, not only the production efficiency is decreased but also the pattern-formed layers may be swollen and dissolved by the influence of the photoresist peeling solution, which is inconvenient. Therefore, it has been a demand for a method in which the photoresist peeling solution can easily act on the photoresist layer in the peeling treatment thereof.